# The free oxygen (O$_2$) in the atmosphere is largely a result of what process?

Does it come from a)atmospheric lightning and ozone dissociation.

b)meteorite impacts early in the earth's geological history.

c)Paleozoic volcanic activity.

d)photosynthetic processes.

e)photodissociation of water molecules (from solar ultraviolet light).

I believe it is D, but not 100%.

Thanks